The present invention relates to electronic circuits and, more particularly, to electronic circuits that regulate current.
Battery powered electronic devices often include components for monitoring the current to and from a battery pack. This is typically done by putting a sense resistor in the path between the battery pack and the load and also between the charging unit and the battery pack. Then, a regulating circuit measures the voltage across the sense resistors and limits the current to or from the battery pack accordingly. This limiting is typically performed by using a transistor in the current path from the battery pack to the load or charging unit and by causing the transistor""s control terminal to be driven in response to the voltage across the sense resistor.
With battery and non-battery powered devices, often a main goal is to lose as little voltage as possible across the regulating circuitry when not limiting current. With typical current regulators, voltage is lost in two places: as current flows through the sense resistor and as current flows through the transistor. Consequently, when not limiting current in order to avoid wasting power or generating heat, it is desired that the resistance of the sense resistor and the resistance of the transistor be as low as possible for the area available for these components.
A transistor exhibits low resistance when its control terminal is held at a voltage appropriate to turn the transistor xe2x80x9cfull on.xe2x80x9d In current regulating circuitry, this is often accomplished through the use of an amplifier coupled to the sense resistor with the amplifier output driving the control terminal of the transistor. With the appropriate choice of sense resistor and amplifier characteristics, the current regulating circuitry can be constructed such that for currents less than a regulation current limit, the amplifier drives the control terminal of the transistor to a full on state. As current through the sense resistor increases above the regulation current limit, the amplifier drives the control terminal of the transistor to reduce the current to the regulation current limit. This has the effect of providing little resistance when current passing through the regulating circuitry is less than the regulation current limit and provides the appropriate amount of resistance through the transistor to limit the current to the regulation current when a load tries to draw an excess amount of current.
Dealing with the resistance of the sense resistor, however, is more problematic. For example, when one desires to have a regulation current limit of one amp and needs to have less than 10% error, a sense resistor in series with a transistor needs to be at least 20 milliohms to provide sufficient feedback to the amplifier. With one amp of current flowing through the sense resistor, 20 milliwatts of power loss is generated. Besides the excess heat that this power loss generates, this amount of power loss is unacceptably high for many applications. Furthermore to build such a resistor on an integrated circuit that can tolerate such a high current requires a great deal of real estate.
In accordance with the present invention, there is provided an apparatus for regulating current. The apparatus uses a current limiter in the current path to or from a power source to limit the current passing through the current path to a selected amount. Instead of using an external sense resistor, the apparatus uses a portion of the resistance inherent in the current limiter to provide feedback to circuitry regulating the current.
In one aspect, an internal conductive material of the current limiter is tapped at one point to provide feedback to circuitry regulating the current.
In another aspect, an internal conductive material of the current limiter is tapped at several points and circuitry is added which allows the feedback provided to the regulating circuitry to be dynamically selected.
Because these aspects of the invention use resistance inherent to the current limiter to provide feedback, the current regulating circuitry is able to avoid the voltage loss associated with current passing through a sense resistor external to the current limiter. Removing the external sense resistor also avoids the heat generated by the current that would pass through it. Eliminating the external sense resistor reduces the amount of chip real estate that would be used to create a resistor capable of carrying a relatively large amount of current, thus decreasing costs of fabrication and allowing other components to be constructed in valuable chip real estate.